The present invention relates generally to optical devices and, more particularly, to optoelectronic devices based on electron tunneling.
The increasing speed of optical communications is fueling the race to achieve ever faster optical communications devices for transmitting, modulating and detecting electromagnetic signals. Terahertz speeds are expected in the near future, and optical communication devices that can operate at such high speeds are in great demand.
A possible approach to achieving high speed optoelectronic devices for use as optical communication devices is electron tunneling. Electron tunneling-based devices, such as metal-insulator-metal (M-I-M) devices for use as infrared and far-infrared detectors and frequency mixers have been explored in the past (see, for example, S. M. Faris, et al., xe2x80x9cDetection of optical and infrared radiation with DC-biased electron-tunneling metal-barrier-metal diodes,xe2x80x9d IEEE Journal of Quantum Electronics, vol. QE-9, no. 7 (1973); L. O. Hocker, et al., xe2x80x9cFrequency mixing in the infrared and far-infrared using a metal-to-metal point contact diode,xe2x80x9d Applied Physics Letters, vol. 12, no. 12 (1968); and C. Fumeaux, et al., xe2x80x9cNanometer thin-film Nixe2x80x94NiOxe2x80x94Ni diodes for detection and mixing of 30 THz radiation,xe2x80x9d Infrared Physics and Technology, 39 (1998)). Such M-I-M devices generally operate on the basis of electron rectification and current production due to incident electromagnetic energy and resulting electron tunneling effects. M-I-M devices can normally be used to rectify extremely high frequencies, extending into the optical frequency range.
In addition to high frequency rectification, it is also desirable to achieve high degrees of asymmetry and nonlinearity in the current-versus-voltage (I-V) curve in electron tunneling devices. The differential resistance of the device, which corresponds to the sensitivity of the device to incoming electromagnetic energy, is directly related to the nonlinearity of the I-V curve. However, prior art M-I-M devices generally exhibit low degrees of asymmetry and nonlinearity in the electron transport such that the efficiency of such devices is limited. A high degree of nonlinearity improves the quantum efficiency of the electron tunneling device, which is number of electrons collected for each photon incident on the M-I-M device. High quantum efficiency is crucial for efficient operation and high sensitivity of the M-I-M diode in all optoelectronic applications. For purposes of this application, a diode is defined as a two-terminal device. A high degree of nonlinearity offers specific advantages in certain applications. For example, in optical mixing, second order derivatives of the current-voltage relationship determine the magnitude of the signal produced in frequency down-conversion. A higher degree of asymmetry in the I-V curve between positive values of V (forward bias voltage) and negative values of V (reverse bias voltage) results in better rectification performance of the device. A high degree of asymmetry is required, for example, to achieve efficient large signal rectification such as in the detection of high intensity incident fields. In this high intensity field regime, the electron tunneling device functions as a switch, and it is desirable to attain a low resistance value in one polarity of voltage and a high resistance value in the opposite polarity of voltage is desired. Alternatively, with low field intensities and large photon energies, the incident field sweeps a larger portion of the electron tunneling device dark I-V curve and, consequently, the high asymmetry translates into high responsivity and as well as high quantum efficiency and sensitivity in electromagnetic radiation detection.
The fabrication of the combinations of alternate layers of metals and insulators in M-I-M-based devices, in comparison to semiconductor materials, is advantageous due to ease of deposition of materials relative to semiconductor fabrication. It has been suggested that the recent trend of decreasing the size of electronic devices to achieve high speed switching will ultimately make semiconductor-based devices impractical due to fluctuation of carrier concentration, which occurs when semiconductor devices are reduced to mesoscopic regimes (see, for example, Suemasu, et al., xe2x80x9cMetal (CoSi2)/Insulator(CaF2) resonant tunneling diode,xe2x80x9d Japanese Journal of Applied Physics, vol. 33 (1994), hereafter Suemasu). Moreover, semiconductor devices are generally single bandgap energy devices. This characteristic of semiconductor devices means that, in detection applications, no current is produced when a photon having energy less than the bandgap energy is incident on the semiconductor device. In other words, the response of the semiconductor device is limited by the bandgap energy. When a semiconductor diode is used to rectify high frequency oscillations, the semiconductor material limits the frequency response of the diode because the charge carriers must be transported through a band, in which concentration are limited in comparison to a metal.
Existing electron tunneling devices based on metal-oxide combinations are generally fabricated by forming a metal layer, exposing the metal layer for a certain amount of time such that the native oxide of the metal is formed, then repeating the process as desired. Photolithography techniques may also be used to achieve desired shapes and patterns in the metals and insulators. For example, Suemasu describes a metal (CoSi2)/insulator(CaF2) resonant tunneling diode with a configuration M-I-M-I-M-I-M triple-barrier structure for use as long wavelength (far-infrared and milliwave) detectors and emitters. However, the M-I-M-I-M-I-M device of Suemasu is much more complicated than the simple M-I-M devices, and must be fabricated using a complex epitaxial growth procedure using exotic materials. In fact, Suemasu chooses to use the triple-barrier structure rather than a slightly simpler double-barrier structure for apparently better performance results in the electron tunneling process. Therefore, although the M-I-M-I-M-I-M device of Suemasu achieves much higher degrees of asymmetry and nonlinearity in the I-V curve than the M-I-M devices, the performance gains come at the cost of the simplicity in design and fabrication.
An alternative approach is the use of a combination of a metal and a semiconductor in a metal-insulator-semiconductor (MIS) configuration (see, for example, T. Yamada, et al., xe2x80x9cSemiconductor Device Using MIS Capacitor,xe2x80x9d U.S. Pat. No. 5,018,000, issued 21 May 1991). The drawback to currently available MIS devices is also the limited efficiency due to asymmetry and nonlinearity limitations. MIS devices cannot operate at as high frequencies as M-I-M devices because the concentration of electron states in the semiconductor is lower than that from a metal.
At this time, infrared detectors, for example, capable of receiving electromagnetic signal at terahertz rates, at room temperature, are not readily available, to the Applicants"" knowledge. Temperature-controlled alternatives, such as narrow bandgap semiconductor detectors, and bolometers, exist on the market, but the extra considerations associated with the temperature control mechanism make such devices expensive and bulky. Prior art M-I-M detectors are capable of detecting infrared radiation without cooling, but these prior art detectors are not sensitive enough for practical applications.
As will be seen hereinafter, the present invention provides a significant improvement over the prior art as discussed above by virtue of its ability to provide the increased performance while, at the same time, having significant advantages in its manufacturability.
As will be described in more detail hereinafter, a number of high speed electron tunneling devices are disclosed herein. The devices of the present invention are especially distinguishable from the aforementioned electron tunneling devices of the prior art by the implementation of resonant tunneling using at least one layer of an amorphous material in the devices. In a first aspect of the invention, a detector for detecting electromagnetic radiation incident thereon is disclosed. The detector has an output, exhibits a given responsivity and includes first and second non-insulating layers spaced apart from one another such that a given voltage can be applied across the first and second non-insulating layers, the first non-insulating layer being formed of a metal, and the first and second non-insulating layers being configured to form an antenna structure for receiving electromagnetic radiation. The detector further includes an arrangement disposed between the first and second non-insulating layers and configured to serve as a transport of electrons between the first and second non-insulating layers as a result of the electromagnetic radiation being received at the antenna structure. This arrangement includes a first layer of an amorphous material and a second layer of material, configured to cooperate with the first layer of the amorphous material such that the transport of electrons includes, at least in part, transport by tunneling, and such that at least a portion of the electromagnetic radiation incident on the antenna is converted to an electrical signal at the output, the electrical signal having an intensity which depends on the given responsivity. For purposes of this application, an amorphous material is considered to include all materials which are not single crystal in structure.
In a second aspect of the invention, an emitter for providing electromagnetic radiation of a desired frequency at an output is described. The emitter includes a voltage source, for providing a bias voltage, and first and second non-insulating layers, which non-insulating layers are spaced apart from one another such that the bias voltage can be applied across the first and second non-insulating layers. The emitter also includes an arrangement disposed between the first and second non-insulating layers and configured to serve as a transport of electrons between the first and second non-insulating layers as a result of the bias voltage. This arrangement is configured to exhibit a negative differential resistance when the bias voltage is applied across the first and second non-insulating layers. The arrangement includes a first layer of an amorphous material and a second layer of material, which second layer of material is configured to cooperate with the first layer of amorphous material such that the transport of electrons includes, at least in part, transport by means of tunneling, and such that an oscillation in the transport of electrons results. This oscillation has an oscillation frequency equal to the desired frequency, due to the negative differential resistance, and causes an emission of electromagnetic radiation of the desired frequency at the output.
In a third aspect of the invention, a modulator for modulating an input electromagnetic radiation incident thereon and providing a modulated electromagnetic radiation at an output is described. The modulator includes a voltage source for providing a modulation voltage, which modulation voltage is switchable between first and second voltage values. The modulator also includes first and second non-insulating layers spaced apart from one another such that the modulation voltage can be applied across the first and second non-insulating layers. The first and second non-insulating layers are configured to form an antenna structure for absorbing a given fraction of the input electromagnetic radiation with a given value of absorptivity, while a remainder of the input electromagnetic radiation is reflected by the antenna structure, where absorptivity is defined as a ratio of an intensity of the given fraction to a total intensity of the input electromagnetic radiation. The modulator further includes an arrangement disposed between the first and second non-insulating layers and configured to serve as a transport of electrons between the first and second non-insulating layers as a result of the modulation voltage. This arrangement includes a first layer of an amorphous material and a second layer of material, which second layer of material is configured to cooperate with the first layer of the amorphous material such that the transport of electrons includes, at least in part, transport by means of tunneling, with respect to the modulation voltage. The arrangement is configured to cooperate with the first and second non-insulating layers such that the antenna exhibits a first value of absorptivity, when modulation voltage of the first voltage value is applied across the first and second non-insulating layers, and exhibits a distinct, second value of absorptivity, when modulation voltage of the second voltage value is applied across the first and second non-insulating layers, causing the antenna structure to reflect a different amount of the input electromagnetic radiation to the output as modulated electromagnetic radiation, depending on the modulation voltage. The modulator is configurable to operate as a digital device, in which only discrete, first and second voltage values are used, or as an analog device, in which continuous values of voltage between the aforedescribed first and second voltage values are used to achieve a continuum of values of absorptivity between the aforementioned first and second values of absorptivity.
In a fourth aspect of the present invention, a modulator assembly for receiving a modulation signal, modulating an input electromagnetic radiation and providing an output electromagnetic radiation is described. The modulator assembly includes a voltage source for providing a bias voltage, and first and second non-insulating layers, which non-insulating layers are spaced apart from one another such that the bias voltage can be applied across the first and second non-insulating layers. The first and second non-insulating layers are also configured to form a first antenna structure for receiving the modulation signal and converting the modulation signal so received into a modulation voltage, which modulation voltage is also applied across the first and second non-insulating layers. The modulator assembly also includes an arrangement disposed between the first and second non-insulating layers and configured to serve as a transport of electrons between the first and second non-insulating layers as a result of the modulation voltage. The arrangement includes a first layer of an amorphous material and a second layer of material, which second layer of material is configured to cooperate with the first layer of the amorphous material such that the transport of electrons includes, at least in part, transport by means of tunneling. The modulator assembly further includes a second antenna structure having an absorptance value, which absorptance value depends on the aforementioned modulation voltage. The second antenna structure is configured to receive and selectively absorb the input electromagnetic radiation in proportion to the absorptance value so as to produce the output electromagnetic radiation.
In an fifth aspect of the present invention, a field effect transistor for receiving an external signal, switching an input signal according to the received, external signal and providing an output signal is described. The external signal is switchable between a first value and a second value, and the field effect transistor includes a diode structure including a source electrode for receiving the input signal and a drain electrode spaced apart from the source electrode such that a given voltage can be applied across the source and drain electrodes. The diode structure further includes an arrangement disposed between the source and drain electrodes and configured to serve as a transport of electrons between the source and drain electrodes. The arrangement includes at least a first layer of an amorphous material configured such that the transport of electrons includes, at least in part, transport by means of resonant tunneling with a given value of a tunneling probability. The field effect transistor also includes a shielding layer at least partially surrounding the diode structure. The field effect transistor further includes a gate electrode disposed adjacent to the shielding layer and is configured to receive the external signal and to apply the external signal as the given voltage across the source and drain electrodes such that, when the first value of external signal is received at the gate electrode, a first signal value is provided as the output signal at the drain electrode and, when the second value of external signal is received at the gate electrode, a second signal is provided as the output signal at the drain electrode.
In a sixth aspect of the present invention, a junction transistor is described. The junction transistor includes an emitter electrode and a base electrode, which is spaced apart from the emitter electrode such that a given voltage can be applied across the emitter and base electrodes and, consequently, electrons are emitted by the emitter electrode toward the base electrode. The junction transistor also includes a first tunneling structure disposed between the emitter and base electrodes. The first tunneling structure is configured to serve as a transport of electrons between the emitter and base electrodes and includes at least a first layer of an amorphous material configured such that the transport of electrons includes, at least in part, transport by means of resonant tunneling with a tunneling probability. The tunneling probability depends on the given voltage. The junction transistor further includes a collector electrode, which is spaced apart from the base electrode, and a second tunneling structure, which is disposed between the base and collector electrodes. The second tunneling structure is configured to serve as a transport, between the base and collector electrodes, of at least a portion of the electrons emitted by the emitter electrode such that the portion of electrons is collected by the collector electrode with a collection efficiency and the transport of electrons includes, at least in part, transport by means of tunneling.
In a seventh aspect of the present invention, an optoelectronic amplification element is described. The optoelectronic amplification element is formed by combining the aforementioned field effect transistor or junction transistor with a detector coupled to the control electrode and an emitter coupled to an output. The optoelectronic amplification element is configured such that electromagnetic radiation incident upon the detector generates a voltage across the detector and subsequently across the control electrodes of the transistor, base and emitter in the case of the junction transistor. The control voltage across the control electrodes of the transistor in turn controls the bias voltage on the emitting device which may be tuned to emit substantially more electromagnetic radiation than the amount initially incident upon the device.
In an eighth aspect of the present invention, an optoelectronic mixer element for simultaneously receiving at least two distinct frequencies of electromagnetic radiation and producing an output signal having a beat frequency, which beat frequency is a combination of said distinct frequencies is described. The optoelectronic mixer element includes first and second non-insulating layers spaced apart from one another such that a given voltage can be applied across the first and second non-insulating layers. The first and second non-insulating layers are configured to form an antenna structure for receiving electromagnetic radiation of the distinct frequencies. The optoelectronic mixer element further includes an arrangement disposed between the first and second non-insulating layers and configured to serve as a transport of electrons between the first and second non-insulating layers as a result of the two distinct frequencies of electromagnetic radiation being received at the antenna structure. The arrangement includes at least a first layer of an amorphous material such that the transport of electrons includes, at least in part, transport by means of resonant tunneling, and such that at least a portion of the electromagnetic radiation incident on the antenna is converted to the output signal having the beat frequency.
In a ninth aspect of the resent invention, an electron tunneling device includes first and second non-insulating layers. The first and second non-insulating layers are spaced apart from one another such that a given voltage can be applied across the first and second non-insulating layers, and the first non-insulating layer is formed of a semiconductor material while the second non-insulating layer is formed of a metal. The electron tunneling device further includes an arrangement disposed between the first and second non-insulating layers and configured to serve as a transport of electrons between the first and second non-insulating layers. This arrangement includes a first layer of an amorphous material such that using only the first layer of amorphous material in the arrangement would result in a given degree of nonlinearity in the transport of electrons, with respect to the given voltage. However, in accordance with a first aspect of the present invention, the arrangement includes a second layer of material, which second layer is configured to cooperate with the first layer of amorphous material such that the transport of electrons includes, at least in part, transport by tunneling, and such that the nonlinearity in the transport of electrons, with respect to the given voltage, is increased over and above the given degree of nonlinearity.
In a tenth aspect of the invention, the first non-insulating layer in the electron tunneling device is formed of a superconductor. The first non-insulating layer in the electron tunneling device can also be formed of a semi-metal or be in a form of a quantum well or a superlattice.
In an eleventh aspect of the invention, the arrangement in the electron tunneling device further includes a third layer of material, which is configured to cooperate with the first layer of the amorphous material and the second layer of material such that the nonlinearity in the transport of electrons, with respect to the given voltage, is further increased over and above the given degree of nonlinearity.